My parent’s CPF-OA has about $7K.
OK. From age 55 that's a $7K "piggy bank" earning 2.5% interest, and that's not a bad thing. It is possible to transfer some/all of those dollars into your parent's Retirement Account where they'd earn at least 4%. Of course then they'll need to stream out as monthly payments, which is not the same as a "piggy bank."
We’re from a lower income family, so not much CPF. Currently Both my parents are staying in a 3-room flat, with probably 50 years of lease left.
OK, that seems really quite reasonable. There is something called the HDB Lease Buyback Scheme which is a good program if they need to raise some funds from the equity in their HDB leasehold (and if they qualify) while still being able to live in their unit. It's something they could investigate to see if it's a good fit. The government heavily promotes that Scheme as a way for elder Singaporeans to boost their retirement income, and it often makes sense.
Bear in mind that each parent separately qualifies for bonus interest on the first $60,000 of combined CPF balances (of which up to $20,000 can be counted from OA specifically). Meaning that usually you/they want to make sure that both of them are maximizing available bonus interest. Let's suppose for example Parent #1 has these balances:
OA: $7,000
MA: $10,000
SA: $3,000
RA: $58,000
and Parent #2 has these balances:
OA: $23,000
MA: $4,000
SA: $2,000
RA: $28,000
Parent #1 is already earning maximum bonus interest. MA+SA+RA is $60,000 or more, and that's good enough to do it. However, Parent #2 in this example isn't earning maximum bonus interest. Parent #2 is earning bonus interest on MA+SA+RA ($34,000) plus $20,000 from OA = $54,000. That's less than $60,000, so there's some bonus interest yet to be earned. Therefore, unless there's a good/strong reason otherwise, Parent #2 should get at least the next $6,000 of Retirement Account top ups because that'll earn 5% interest (4% base interest plus the 1% bonus interest), a higher interest rate than Parent #1 would earn on the same top up. Also, in this example Parent #2 has quite a lot of Ordinary Account dollars earning only 2.5% interest relative to Retirement Account dollars, so it's probably a good idea to transfer some OA dollars into RA.
In topping up my parent’s CPF-RA, I am also thinking of transiting to the CPF life at later age (say 69) so it would give me a few more years to top up the RA account every year.
That's certainly allowed, but you (and anybody else) is still allowed to top up a CPF Retirement Account even after CPF LIFE payouts start. The only top up limit is the current Enhanced Retirement Sum, based on principal only. (Interest doesn't count toward this particular limit.)
When you top up a CPF Retirement Account after CPF LIFE payouts start, there are two choices. If the member does nothing, then the top up gets paid out as "Additional Monthly Payments" (AMPs) starting in the July following the top up. AMPs are similar to the classic Retirement Sum Scheme, so they only last for a fixed term and aren't paid for life. The other choice is to contact the CPF Board any time at least a month or two before that next July and ask for an increase in the CPF LIFE payout amount. The CPF Board will recalculate the CPF LIFE payout amount (based on the current CPF LIFE payout plan) and start paying the increased amount the next month.